This application claims priority of Taiwan Patent Application No. 90212813, filed on Jul. 27, 2001.
1. Field of the Invention
The invention relates to an illuminating device, more particularly to an illuminating device that is adapted to provide a light output with a predetermined polarization state to a projection display, thereby achieving a higher light utilization rate.
2. Description of the Related Art
Light is a kind of electromagnetic wave that, in addition to having the characteristics of direction of travel, frequency, and phase, etc., also has a polarization. Light with a fixed direction of electric field oscillation and a direction of magnetic field oscillation is known as polarized light. Where the polarized light has an electric field direction parallel or perpendicular to an incident light ray, the same is referred to as P-polarized light or S-polarized light. As polarized light occurs only in a plane and can be interpreted by using mathematical models, it provides facility in use. Therefore, in a conventional projection display system, a light polarization state converter is often employed to convert the polarization state of light from one to the other for optimum utilization of light rays.
Referring to FIGS. 1 and 2, a conventional illuminating device 1 for a projection display system is shown to include a light source 11, a light integrator 12, a polarization state converter 2 and a lens unit 13. The light source 11 includes a reflector 111 and a light bulb 112. The reflector 111 is disposed to reflect the light rays emitted from the light bulb 112 toward the light integrator 12 for passage through the polarization state converter 2 and the lens unit 13 for subsequent projection on a display panel 14. The polarization state converter 2 in the conventional illuminating device 1 includes a reflective seat 21 and a plurality of half-wavelength plates 22. The reflective seat 21 has a light input side 211, a light output side 212, and a plurality of first and second plated films 213, 214 alternately arranged and interposed between the light input side 211 and the light output side 212. The light input side 211 faces the light integrator 12, and is provided with a plurality of light shielding regions 215 that are vertically spaced apart from each other. The light output side 212 faces the display panel 14. The first and second plated films 213, 214 are disposed parallel to each other and to incline downwardly from the light input side 211 to the light output side 212. The half-wavelength plates 22 are disposed on the light output side 212 facing the display panel 14 such that they are vertically spaced apart from each other. The half-wavelength plates 22 correspond in position and length to reflective portions of the first plated films 213, respectively.
When S-polarized light is employed for displaying images in the conventional projection display system, light of both the P-polarization state and the S-polarization state from the light source 11 radiates toward the light integrator 12 and the polarization state converter 2. When light of the P-polarization state reaches parts of the light input side 211 that are not shielded by the light shielding regions 215, it will pass directly through the first plated films 213 and the half-wavelength plates 22, and is converted into light of the S-polarization state for subsequent projection onto the display panel 14. However, when light of the S-polarization state reaches the first plated films 213, it will be reflected by the latter toward the second plated films 214 for further reflection so as to pass through parts of the light output side 212 that are not provided with the half-wavelength plates 22 for projection onto the display panel 14. Accordingly, when P-polarized light is adopted for the conventional projection display system, the half-wavelength plates 22 will be disposed to correspond in position to reflective portions of the second plated films 214 so as to convert the S-polarized light radiated from the light source 11. A further discussion thereon is dispensed with herein for the sake of brevity.
In the conventional illuminating device 1, in order that the light to be projected onto the display panel 14 can have the same polarization state, about one-half of the area of the light input side 211 of the reflective seat 21 has to be provided with the light shielding regions 215 so that the light passing through the light input side 211 can be effectively converted for utilization. Although the construction of the aforesaid device 1 can ensure that the light outputted to the display panel 14 has the same polarization state, about 50% of the light emitted from the light source 11 is wasted. It is therefore evident that although the polarization state converter 2 is capable of providing light of the same polarization state, the light conversion efficiency is low, and the luminosity of the converted light is relatively poor.
Therefore, the main object of the present invention is to provide an illuminating device that is adapted to provide a light output with a predetermined polarization state to a projection display and that is capable of overcoming the aforesaid drawbacks of the prior art.
Accordingly, an illuminating device of the present invention is adapted to provide a light output to a projection display, and includes a light source, a light guiding pipe, a quarter-wavelength plate, a reflective polarizer, and a reflector. The light source generates output light that converges at a focal point. The light guiding pipe is aligned with the light source along an optical axis, and has a reflective surrounding wall surface that defines a passage. The passage has a light input end proximate to the light source, and a light output end distal from the light source. The focal point is coincident with a plane of the light input end such that the output light from the light source can enter into the passage via the light input end, can be guided by the surrounding wall surface toward the light output end, and can exit from the passage via the light output end. The quarter-wavelength plate and the reflective polarizer are disposed in sequence relative to the passage of the light guiding pipe such that the light that travels in a direction from the light input end to the light output end of the light guiding pipe passes through the quarter-wavelength plate before reaching the reflective polarizer. The reflective polarizer allows light of a first polarization state to pass therethrough, and reflects light of a second polarization state back into the passage of the light guiding pipe via the light output end. The reflector is mounted on the light guiding pipe at the light input end of the passage, and has an outer surface that faces the light source, an inner surface that faces the light output end of the passage, and a through-hole that extends through the outer and inner surfaces of the reflector and that is centered at the focal point to permit entry of the output light from the light source into the passage. The reflector reflects the light that travels in the light guiding pipe in a direction from the light output end to the light input end back to the light output end. The light of the second polarization state that was reflected by the reflective polarizer is converted into the light of the first polarization state when passing through the quarter-wavelength plate for a second time.